It's just a rumor, but it sounds pretty solid: Superman baddie Brainiac may be the villain of the big Justice League movie.
Brainiac has taken many, many forms over the years, tracing his origins way back to 1958. He's been an alien, he's been cloned, he's inhabited a robot body, he's been an entire army of Brainiac drones and more. He even has "descendants" in the future, some of whom are superheroes instead of villains.
Latino Review reports that Argo scripter Chris Terrio is writing Justice League, just as he did Batman v. Superman: Dawn of Justice, with Zack Snyder directing both. The report suggests that Brainiac is Terrio's choice of supervillain to face the team of heroes, and he certainly fits the bill.
In a special podcast, Latino Review's "El Mayimbe" revealed that he's heard Brainiac is traveling the universe when he detects the Kryptonian beacon that brought Zod to Earth in Man of Steel. His arrival on Earth is the reason the Justice League is drawn together to fight a single foe. El Mayimbe also says that Darkseid — DC's ultimate villain equivalent to Marvel's Thanos — will still come to DC's cinematic universe, but it won't happen for a while.
Just like Marvel, DC has come up with a long-term plan for its superhero films, and has reportedly already nailed down release dates for nine future films, though none of those dates have yet been revealed. Justice League is the only one to be officially confirmed as one of the nine, and is expected in 2018 or '19. Other rumored possibilities include Shazam with Dwayne Johnson starring as villain Black Adam, a Wonder Woman solo movie starring Gal Gadot, an Aquaman flick for Jason Momoa and maybe even a Suicide Squad film.
Also interesting to note: much like the similarities between Thanos and Darkseid, one could easily draw parallels between Ultron (the villain of Avengers 2) and Brainiac.